The present invention relates to systems for steering and for trimming a marine drive unit and, more particularly, to a single combined system for the operation of hydraulic power steering and trim systems.
A power steering system for a typical transom-mounted marine drive unit utilizes a hydraulic assist to reduce steering loads to the operator. The steering arm of the marine drive, which pivots the drive unit about a generally vertical axis, is connected to the piston rod of a double-acting hydraulic cylinder and the cylinder housing is attached to the boat. The steering wheel of the boat is connected through a steering cable to a movable valve that controls the direction and volume of hydraulic fluid supplied to the cylinder. The operator of the boat thus needs only to exert enough effort to operate the valve and not the total effort required to actually steer the boat.
A power trim system for a marine drive also employs a double-acting hydraulic cylinder or cylinders to pivot the drive unit about a generally horizontal axis to adjust the drive angle of the propulsion unit with respect to the boat for on-plane and off-plane operation. Fluid power to operate the cylinder to raise or lower the drive unit is provided by a reversible pump, such as a gear pump, and controlled by a shuttle valve which is operated by pump pressure and direction.
Typically, a power steering system and a power trim system each utilizes a separate hydraulic pump to generate the required fluid pressure. Power steering system pumps are, further, typically driven by the engine used to power the boat. Trim systems, on the other hand, most commonly use a separate electric motor to drive the trim system pump. Using the boat engine to drive the hydraulic pump for the power steering system helps assure that the system will be operative for steering whenever the boat is moving under the power of the engine. A separate electric motor to power the trim system pump offers the advantage of permitting the drive unit to be trimmed or tilted up out of the water when the engine is not running.
Pending U.S. patent application Ser. No. 079,097, filed July 29, 1987 and assigned to the assignee of the present application, discloses a power steering system which uses an electric motor to drive the hydraulic pump. In that system, a hydraulic accumulator is disposed between the pump and the cylinder control valve to provide the direct source of fluid pressure for the system. The accumulator is periodically pressurized by automatic activation of the electric motor and pump to maintain the fluid pressure in the accumulator and available to operate the power steering between selected maximum and minimum limits. Use of an accumulator also eliminates the need to have the hydraulic pump and motor operating continuously.
U.S. Pat. No. 4,490,120 shows a hydraulic system for trimming and tilting an outboard motor in which a single electric motor driven pump powers a pair of hydraulic trim cylinders and a single hydraulic tilt cylinder. U.S. Pat. No. 4,050,359 describes a system in which a pair of hydraulic cylinders provides combined trim and tilt functions, the latter to pivot the outboard motor to its highest position as for trailering. U.S. Pat. No. 3,915,111 discloses a hydraulic marine propulsion device in which the trim/tilt system and the power steering system are each operated by a separate hydraulic pump.
Thus, basic differences between trim/tilt systems and steering systems, such as the required operating pressures and the nature of the duty cycle, have lead to substantially different system design and operation. For example, a trim system typically utilizes a reversible gear pump, whereas a power steering system may typically use a high volume, unidirectional device, such as a vane pump.
Despite the differences in the two systems described above, it would be desirable to utilize a common fluid pressure supply for the combined operation of the trim/tilt and power steering systems in a boat.